It is well known to apply Berek-type, and Non-Berek-type Plate retarders to enter phase delays between orthogonal components of a polarized beam of electromagnetism caused to pass therethrough, where Berek-type retarders are characterized by a Fast Axis thereof being ideally oriented perpendicular to surfaces of said Plates, and Non-Berek-type retarders are characterized by a Fast Axis being ideally oriented parallel to said surfaces. Often in the prior art two Berek-type Plates are tipped with respect to one another such that an incident beam of electromagnetic radiation enters the first side of a first plate and is refracted thereby so that it exits the second side thereof at a location offset from the incident beam, which beam exiting the second side of said first Plate then passes through the second Plate in a similar manner, where it is refracted such that the beam exiting from the second side thereof is essentially non-deviated and not offset from the original incident beam that entered the first side of said first Plate. However, the phase angle between the orthogonal components of said electromagnetic beam entering the first side of the first Plate, is changed by
passing through the system of two tipped Berek-type Plates. And often as well, two Non-Berek-type Plates are positioned with respect to one another such the surfaces thereof are substantially parallel to one another, and Fast Axes thereof are rotated about an axis that passes perpendicularly through said Plate surfaces. Again the beam exiting from the second side of a second plate in the succession of Plates is essentially non-deviated and not offset from the original incident beam that entered the first side of said first Plate. And, again, the phase angle between the orthogonal components of said electromagnetic beam entering the first side of the first Plate, is changed by passing through the system of at least two Non-Berek-type Plates which are rotated with respect to one another such that their Fast Axes are not in-line with one another.
The use of only multiple sequential Berek or Non-Berek-type retarders exclusively is conventional and is outside the scope of the present invention. The present invention however, while still involving use of at least two Plates in succession, said Plates each having two surfaces that are parallel to or substantially parallel to one another, differs in that in at least one, (preferably all) Plates present the Fast Axis is neither Perpendicular nor Parallel to said parallel or substantially parallel surfaces. Instead the Fast Axis of at least one Plate is at what is termed herein as a “Biased angle” which is an angle between 0.0 and +/−90 degrees, (excluding of course 0.0 and 90 degrees per se.), as related to the surfaces of and Fat Axis of Berek-type Plates.
A particularly relevant Patent is that to Herzinger et al., U.S. Pat. No. 5,835,222. This Patent teaches how to determine the orientation of a Fast Axis in a Retarder Plate.
A problem the present invention addresses is that in conventional Retarders constructed from purely Berek or Non-Berek-type Plates are limited as to the range over which they can provide significant retardance between orthogonal components of an electromagnetic beam. A need remains for a Retarder that can, for instance, provide significant retardance over a range of about 0.3 to 3.0+ microns.